Hydrostatic transmissions have been used to propel vehicles, such as zero-turn mowers. Hydrostatic transmissions typically include a variable displacement main pump connected with a hydraulic motor in a closed hydraulic circuit via a high pressure supply line and a low pressure return line. The main pump is typically driven by a prime mover, e.g. an internal combustion engine of the vehicle. Changing the displacement of the main pump changes the output flow rate, which controls the speed of the motor connected with the main pump. Main pump outflow can also be reversed, thus reversing the directional rotation of the hydraulic motor attached to the main pump. Both acceleration and deceleration of the transmission are controlled by varying the displacement of the main pump from its neutral position. A charge pump is typically added to the hydraulic circuit to introduce fluid into the circuit to make up for lost fluid due to internal leakage.
Closed hydraulic circuits are known to include shuttle valves in the low pressure line that allow a small portion of working fluid to drain from the low pressure side of the circuit to carry out some of the contaminants and heat. This drained fluid is cooled and cleaned through hydraulic filters and reservoir (cooler) and reintroduced into the circuit by the charge pump.
Bypass valves, which are positioned between a high pressure supply line and a low pressure return line that connect the main pump to the motor, allow the transfer of fluid from one line to the other. The bypass valve can enable the motor to rotate with little resistance, which allows the vehicle to be easily towed.
Known hydraulic systems separate the bypass valve and the flushing valve into separate valve assemblies. In known systems, these two valve assemblies are located in two separate locations within the circuit. This requires the manufacture of two separate components and two separate connection locations within the hydraulic circuit. Where the bypass valve and the separate flushing valve are both found in a pump housing, the two separate valves take up a lot of space and require more material to provide for connection between the valves and the working components of the pump.